Two new techniques using fluorescence are proposed for immunoassays. Based on either shadowing or fluctuations, they can in principle determine the extent to which fluorescent-labelled molecules are bound to large carrier particles or are free in solution. Hence, a measurement of total fluorescence intensity in the forward direction in one case, or of r.m.s. fluctuations in fluorescence intensity in the other, will yield a quantitative measure of bound vs. free antigen or antibody (depending on the specific reaction mode employed). The motivation for using fluorescence labelling is to supplement or replace the radioimmunoassay (RIA), which involves the use of radioactive reagents, expensive equipment and labor-intensive preparation steps.